1. Field of the Invention
The invention relates to a process for increasing the interference suppression, especially in optical proximity switches, with the help of an optical transmitter emitting an optical pulse train, with the help of an optical receiver emitting a measuring signal and with the help of a control and evaluation unit controlling the optical transmitter and evaluating the measuring signal, in which the measuring signal of the optical receiver consists of the superposition of an interference background constantly emitted and a measuring pulse emitted when receiving an optical pulse, and in which the measuring signal of the optical receiver is evaluated by the control and evaluation unit in a signal interval during a measuring pulse and in at least one background interval outside the measuring pulse.
2. Description of Related Art
Optical proximity switches, which operate according to the above-described pulse process, are usually subdivided into three categories. On the one hand, a distinction is made between optical proximity switches, which detect the presence of an object by the absence of optical pulses reflected on a reflector, so-called reflective light barriers, and the optical proximity switches which detect the presence of an object exactly by the reflection of otherwise nonreflected optical pulses on the object to be detected, so-called reflective light sensors. On the other hand, mention is made of optical proximity switches of the so-called one-way light barrier type in which the optical transmitter and the optical receiver are arranged separately, in contrast to the reflective light barrier and reflective light sensor type sensors. In the case of these one-way light barriers, the proof of a close-by object takes place, of course, by the absence of one or more optical pulses, and in this respect, is analogous to the reflective light barrier sensors.
The optical proximity switches known at this time operate mainly according to the so-called pulse process. In this pulse process, the optical transmitter emits a short, intensive light pulse of typically 10 .mu.s to 20 .mu.s length. Then, a pause of, currently, about 500 .mu.s length takes place. To suppress interference, the receiving of further signals is prevented in the case of reflective light sensors and reflective light barriers during the pause between two optical pulses or in the case of one-way light barriers during a part of this pause. This pause is referred to as so-called interference blanking.
The requirements on modem optical proximity switches now essentially are to assure a higher operating frequency with constant or even increased sensitivity..
To meet these requirements, various processes for increasing the interference suppression have been proposed, for example, see German Patent No. 35 30 011. The causes of the interference background superimposed on the measuring signal can be of diverse nature. On the one hand, interference of outside light is produced and in this case is coupled by the optical receiver in the optical proximity switches. But other effects can also play a role, e.g., starting pulses of other devices, alternating-current hums or similar effects, which then are launched capacitively, inductively or galvanically in the electronic structure of the optical proximity switch.
Especially low-frequency interference has a high portion because of the known 1/f-decrease in the noise spectrum. A suppression of this interference would be possible only with averaging times that have a significantly lower time constant than the interference itself. Such an averaging would thus be connected with an extreme reduction of the operating frequency of the optical proximity switch, which, of course, is not desired.
The above-mentioned German Patent 35 30 011 discloses a process for increasing the interference suppression. In this process, especially the effects of stray light are offset. For this purpose, the stray light signal present at the time is stored shortly before the occurrence of each measuring pulse. When the measuring pulse occurs, the signal present at the time and consisting of the stray light signal and measuring pulse is then again stored. The intensity of the actual measuring pulse is then obtained in that the stored signals are subtracted from one another.
This process has some considerable deficiencies. On the one hand, because of the short successive measurements for an input amplifier stage of the optical proximity switch, a high band width is required, which again makes the optical proximity switch sensitive to high frequency interference, such as starting pulses or radio interference. On the other hand, the process described in German Patent 35 30 011 is not suitable for use in one-way light barriers, since there a synchronization of the receiving of the interference background to the measuring pulse is very expensive. Further, systematic spurious components in the interference background increasingly occur exactly near the optical pulses or the measuring pulses, which are caused in the operating pulse for emitting the optical pulse.